The blades of a scissor has a slight bow, this is called the set of a scissor. To demonstrate this hold the scissor in a closed position up to the light and you should see light between the blades.

In order for the blades to perform when cutting, there has to be tension in the blades. If the scissor is too loose and has no tension it will not cut properly, it will simply bend the hairs or the hairs will slide or run on the blades (pushed), resulting incorrect hand movements which can injure your wrist, elbow, shoulder and neck.

You should feel some resistance when you open and close your scissors.

What is the correct angle for setting the tension?

There is not one correct answer, it depends on the quality and manufacturing of the scissor. With high quality scissors, the angle should be more open like in picture no 1 to the right.

Picture 2 to the right is an example of an incorrectly set tension.

The blades of a correctly adjusted scissor will perform together without any side movement or strain on the thumb. This will reduce on-going maintenance and add longevity. Well maintained, good quality scissors cut better and are kinder on the hands.